1. Field of the Invention
My invention relates generally to dental instruments, and more particularly to an apparatus of relatively simple construction and operation for simplistically loading and selectively dispensing into a tooth cavity measured increments of all or a portion of one or two pre-determined quantities of a dental filler material, while maintaining sterility of the part of the apparatus which is introduced into a patient's mouth and avoiding malfunctioning of the apparatus due to the filler's clogging its passageways.
2. Background of the Prior Art
Tooth cavities are filled with various filler materials, most often an amalgam of mercury and silver or mercury and a mixture of silver and tin, and/or copper, and/or zinc. The amalgam preferably is mixed in individual units called "spills". The units most commonly used by the dentist contain either one, two or three spills (the respective quantities consisting of 500 mg., 1000 mg., or 1500 mg). As a rule, it requires up to two spills to fill the average tooth cavity.
The amalgam is extruded from a dental amalgam carrier into a tooth cavity. The period during which the amalgam remains in a plastic state so that it can be extruded from the carrier into a tooth cavity, and compacted in the cavity so as to expel any trapped air and excess mercury is about 5 minutes. Normal dental practice is to introduce small increments of amalgam into the cavity initially, each time tamping the material into all of the cavity's crevices, and then add larger quantities until the cavity is entirely filled.
The properties of amalgam are unique. Being a eutectic mixture, the material has the ability to be transferred from a plastic, semi-solid state to a final solid state in which it can withstand compressive forces over 65,000 psi. The effective working time from the moment the amalgam is completely mixed until it is initially set is approximately three or four minutes. In that time the material passes through three stages, as follows:
(1) Plastic, semi-solid with a high degree of workability; PA1 (2) Crumbling mixture which is of course texture and has a low degree of workability; and PA1 (3) Set, gradually changing from a very low degree of workability to no workability at all.
Within a time frame of 1-2 minutes the mixed amalgam must be placed within the tooth cavity, then compacted and condensed so as to drive off any excess mercury.
The first stage lasts for about 45 seconds to 11/2 minutes. It is essential that the amalgam be transferred from the vessel in which it is mixed to the tooth cavity in the shortest possible period of time. In the past this transfer was done by the dentist using his fingers. Later, tubular or cylindrical amalgam carriers were substituted for the fingers.
Some of the earlier forms of amalgam carriers and dispensers were designed to load the carriers through the ends of the instruments' cylindrical or tubular barrel from which the amalgam later is expelled into a tooth cavity. In other words, the material is loaded and dispensed through the same end of the instrument. Such designs have inherent limitations as to the quantities of amalgam which can be carried by the instrument, for if the diameters of the barrels are increased to any significant degree to accommodate more amalgam material the cross-sections of the ends of the barrels would be too large to effectively insert the amalgam into the much smaller diameter of a tooth cavity. The alternative of lengthening the barrels to hold more amalgam is not satisfactory as it would tend to increase the likelihood of the amalgam losing its plasticity and clogging the instrument barrel, in addition to making it next to impossible to load the amalgam mix into the barrel.
A prior art improvement over the "front end" loading design is the provision of a magazine or amalgam-containing chamber situated behind the discharge end of the amalgam dispenser. A plunger is provided in this construction by which the dentist is able to push amalgam out of the magazine through a nozzle from which the amalgam is ejected into a tooth cavity. The magazine is so constructed as to resemble two parallel shafts, but with only one discharge nozzle and with a single plunger. To line up the plunger so that it is axially movable in first one shaft and then the other it is necessary to move the chamber back and forth. This design has certain inherent disadvantages in that the movement of the chamber, the double movement which must be imparted to the plunger, the limitation of the amount of amalgam in the magazine to a single charge, all can cause the instrument's user problems in its operation. In addition, the device's nozzle can become clogged with amalgam, and when this occurs it becomes so difficult to remove the hardened amalgam from the nozzle that quite often the instrument has to be discarded.
The limitation of prior art devices as to the quantity of amalgam which they can carry is a troublesome problem to the dentist. The amount of amalgam which has to be prepared for introduction into a tooth cavity is a relatively large mass in comparison with the mass of filling material that actually can be introduced by the carrier to the tooth cavity at any instant. If a carrier with a magazine having a single chamber is employed, putting all of the required amount of the amalgam mix into the carrier chamber at one time does not solve the problem since it is not possible to compress amalgam from a chamber of larger diameter (i.e. from a larger mass of amalgam) into a carrier exit opening of small diameter (i.e. to a smaller mass of amalgam). This is due to the physical nature of the material. It should be noted that the smaller diameter, which is the diameter of the nozzle's working tip of the instrument that actually is inserted into the tooth cavity, generally cannot be useful if it is any larger than 3.5 mm.
Still another prior-art amalgam carrier provides two plungers situated parallel to one another and each movable axially the length of the apparatus, one for dispensing amalgam from a magazine into a discharge nozzle and the other for expelling the amalgam out of the nozzle and into a tooth cavity. This device is impractical because the amount of material expressed from the nozzle in relationship to that contained in the magazine, and the time required for all of the material to be discharged, are both factors that cause the material in the instrument to become so hardened as to make it difficult to clean the instrument or reload the same instrument quickly enough for re-use by the dentist on the same patient.
In my own dental practice I have seen and/or used one variation or another of these and other prior art devices for conveying dental amalgam to a patient's tooth, and have found them to have numerous shortcomings. It was in seeking to overcome these shortcomings that I devised my present invention which I shall now describe with reference to the attached drawings.